Revealing the Truth About “Trapped Rainbow” Storage of Terahertz Waves in Plasmonic Grating

Graded grating has been commonly used to trap the plasmonic surface waves of different frequencies, called “trapped rainbow.” The physical origin is believed that the group velocity of the surface waves is decreased to zero at the trapping point because the surface wave vector can be increased to an infinite value by the graded grating. However, this theory needs to be further modified according to our results. We demonstrated the surface terahertz (THz) waves near the cutoff frequency of a grating could not be coupled into the propagated modes, but are reflected back, which means the surface THz wave vector cannot be increased to an infinite value. Hence, the trapped rainbow is actually only a kind of reflection. Furthermore, one single deeper groove is enough to reflect most of the surface THz waves, which could be an easier way to control the reflection and transmission of the surface THz waves and realize the compact integrated slowing waveguide, band-pass filter, surface THz wave switch, and reflector.